The identity and function of the virus specific proteins involved in transcription and replication of negative strand RNA viral genomes and the interactions between these two pathways of RNA synthesis are being studied by an analysis of temperature sensitive mutants of vesicular stomatitis virus New Jersey serotype. This analysis comprises an integration of two basic approaches: 1) chemical analyses to identify the structurally altered protein and 2) refined in vitro functional analyses at the level of the individual protein subunits. In addition, the temperature sensitive mutants in conjunction with defective interfering particles are being exploited in attempts to develop a system for isolating the replication pathway in vivo and in vitro.